Plasticizers are incorporated into resins to increase their flexibility, workability, and dispensability. Phthalates, especially, the high molecular weight phthalates (HMWP), are used as plasticizers in PVC. Alternative to phthalates are desirable due to environmental, legislative and regulatory concerns. In particular, the uses of phthalates as plasticizers are under severe pressure. Hydrogenation of phthalates produces 1,2-cyclohexyl dicarboxylates, hereinafter also referred to as cyclohexanoates, which can also be used as plasticizers.
Previous research showed that catalysts consisting of noble metals supported on alumina (Al2O3) with low surface areas are active for the hydrogenation of phthalate to cyclohexanoates. U.S. Pat. No. 5,936,126 (BASF) discloses the hydrogenation of phthalates to cyclohexyl dicarboxylates using catalysts consisting of Ru supported on low surface area alumina at 80° C. to 120° C. and under 10-20 MPa (100-200 atmospheres) pressure.
It has also recently been discovered that materials consisting of noble metals, such as Ru, supported on a silica (SiO2) support with “remnant structure” produced by deposition of an organic ruthenium compound on a silica support to form an organic ruthenium complex on or in the support, followed by decomposition of the complex, have much higher activities and stabilities in the phthalate hydrogenation than reported Ru/Al2O3 catalysts. WO 2004/046076, WO 2004/045767 and WO 2004/046078 (ExxonMobil) disclose catalysts of Ru on silica supports prepared with the remnant structures. However, diffusion rates of bulky reactants such as phthalates through catalysts are slow with most of the chemical reactions taking place on the surface of the catalyst. Accordingly, only a proportion of the ruthenium deposited on a porous silica support is at effective catalytic sites, with metal sites in the center of the support making little contribution.
It is known to produce catalysts that have a shell or rim structure in which particles of support material are provided with noble metal coatings, also called “shell” catalytic materials. For example, U.S. Pat. No. 5,422,329 (Hoechst) discloses a supported catalyst for the production of vinyl acetate from acetic acid, ethylene and oxygen, prepared by impregnating particles of a silica support with an atomized solution of palladium and other noble metal salts to coat the particles. U.S. Pat. No. 5,622,908 (Hoechst) discloses similar catalysts prepared by spraying the support particles with an atomized solution of the noble metal salts. U.S. Pat. No. 5,498,590 (Degussa) also discloses the preparation of a fixed bed catalyst support for vinyl acetate synthesis, in which pyrogenic silica support is sprayed with an atomized impregnating solution comprising a precursor of a noble metal catalytic component to form a shell coating. EP 1 775 021 (Honda) discloses a method of producing catalysts for cleaning emissions from internal combustion engines in which catalyst nanoparticles are deposited on a porous filter support.
Evenly dispersing noble metals onto the surface of silica particles has been found to be problematic using common dispersion techniques. Typically, multiple coating steps are needed to fully coat the surface, and substantial quantities of the noble metals penetrate into the body of the support material where they are inaccessible to the phthalate molecules and are thus unavailable for catalyzing phthalate hydrogenation. US 2012/0296111 (BASF) discloses an egg-shell catalyst for hydrogenating carbocyclic aromatic compounds, such as phthalates, comprising a noble metal, such as ruthenium, deposited on a silica support material. The silica support is in the form of shaped bodies of from 0.5 mm to 25 mm, such as in the form of spherical particles of 1.0 mm to 6.0 mm in diameter. At least 90% of the pores present in the catalyst have a pore size of from 6 to 12 nm. The catalyst is prepared by impregnating a solution of a noble metal salt, such as ruthenium acetate, and an organic acid dispersion aid onto the silica support, for example, by spraying. The ruthenium in the egg-shell catalyst is described as being present down to a penetration depth of 200 μm and the metal dispersity is described as being from 30% to 60% as measured using DIN 66136. Despite the use of repeated impregnation steps to disperse the ruthenium onto the surface of the silica support material, it has been found that the metal is not well dispersed in the catalyst prepared by the method of US 2012/0296111, resulting in a low benzene hydrogenation activity (BHA).
As a consequence, there remains a need for improved metal oxide-supported noble metal catalysts which are highly active in hydrogenation of aromatic compounds, in particular, that allow rapid conversion of phthalates to cyclohexane carboxylic acids and carboxylates. In particular, there is a need for a hydrogenation catalyst that can both facilitate the mass transfer of large aromatic molecules, such as phthalates, during catalytic reactions and make efficient use of the expensive noble metals. There also remains a need for a more efficient method of dispersing a noble metal, salt such as ruthenium, onto the surface of a silica support, especially a small particle size silica support.